I need help with my amps.

See, I took the head out of my old practice amp, because it was a really blown sound and I wanted to run it through my larger amp for distortion. But the only out on the practice amp head is through the headphone jack. I have ran a cable from that to the input on my larger amp, but the sound level is greatly reduced. But then I got to wondering if this could harm my larger amp since the headphone jack is powered. My question is; is there a way to run my little head through my big amp?